I’m having a really odd issue with my e‑fatbike (Bafang M400 mid‑drive). When I’m on the two largest cassette cogs (lowest gears), the motor briefly cuts power once per crank revolution. It’s a clean on‑off “tick,” almost like the system thinks I stopped pedaling for a split second.
I first noticed this after switching from a 38T front chainring to a 30T. At that point it only happened on the largest cog, never on the others.
I figured it might be caused by the undersized chainring, so I put the original back in and swapped the original 1x10 drivetrain for a 1x11 and went from a 36T largest cog to a 51T. But no - the issue still persists. Now it happens on the largest two cogs. Whether I’m soft‑pedaling or pedaling hard against the brakes doesn’t seem to make any difference. It still “ticks” once per revolution.
I’m out of ideas at this point. Torque sensor, maybe? I have another identical bike with a 1x12 drivetrain and an 11–50T cassette, and it doesn’t do this, so I doubt it’s a compatibility issue. Must be something sensor‑related? With the assist turned off everything runs perfectly, so it’s not mechanical.
EDIT: Upon further inspection it seem that the moment the power cuts out seems to perfectly sync with the wheel speed magnet going past the sensor on the chainstay so I'm like 95% sure that a faulty wheel speed sensor is the issue here. I have a spare part ordered so I'm not sure yet but unless there's a 2nd update to this then it solved the issue.
Yeah that part really is crazy. Obviously there's some mechanism but I don't know what. You've alreade checked change in torque and cadence as a cause. The only other thing that comes to mind is the force pointing backwards, experienced by the spindle from the chain pulling on it, which changes depending on which gear you're in and how tensioned the rear derailleur is. But I would think any such effect would be drowned by the forces of your legs on the cranks.